1. Technical Field
This invention relates generally to rf circuitry, and more particularly to a tuning element for planar rf circuits operating in the microwave, millimeter wave, and submillimeter wave range.
2. Background Information
Planar rf circuits find many uses at frequencies of 1-1000 GHz or more. But those frequencies can complicate circuit design because the circuit designer may lack sufficiently well-characterized devices, accurate knowledge of materials properties, and well-developed calculational techniques. So design often proceeds somewhat empirically with the inefficiencies and frustrations of trial and error.
To better visualize the problem, consider a typical planar rf circuit used with a mixer, oscillator, low-noise amplifier, coupler, phase-shifter, or the like, for application in radar, communications, or microwave test equipment. Commonly formed on a substrate and referred to as a microwave integrated circuit (MIC) or monolithic microwave integrated circuit (MMIC), the planar rf circuit usually includes rf tuning elements in the form of transmission lines. They may be microstrip lines, coplanar lines, or slotlines formed on the same substrate, and the circuit designer uses them as distributed rf tuning elements to optimize circuit performance.
In other words, the circuit designer adds lengths of open-circuited or short-circuited transmission line (i.e., tuning stubs) at selected points in the circuit in order to introduce impedances that improve circuit performance. Although the designer can add a wide range of complex impedances in that way to fine tune the circuit, he must know materials parameters such as dielectric constants, absorption coefficients, and metallic conductivities to design the transmission lines and make the design repeatable. In addition, he must use active devices for which the high-frequency electrical response must be accurately characterized and he needs accurate calculational design tools. Such requirements are usually difficult if not impossible to meet and so they severely limit the design process. It is therefore desirable to have a better way to tune planar rf circuits.